Projectors are display devices and can be used to produce an image on many surface types. Multiple projectors are commonly used to increase the size of a projection onto a projection surface whilst retaining high resolution and brightness. For example, four projectors can be arranged in a grid configuration to produce a single image that is four times larger than the image produced by a single projector.
A problem of such multi-projector systems is the geometric alignment of the projected images on the projection surface. It is important that a viewer perceives a single image that has no visible seams or brightness fluctuations. Precise alignment of the projected images is therefore very important. Many current multi-projection systems require a significant amount of manual effort to perform alignment. Some prior art systems perform an automated alignment procedure at system installation time, for example using projected calibration patterns or structured light patterns, commonly known in the art. However, multi-projector systems may fall out of alignment over time, for example, due to physical movement of a projector or surface, building vibration, or heat fluctuations causing small movement of the internal components of one or more of the projectors of the system. When such systems become unaligned, the manual or automated alignment procedure typically needs to be re-run.
Other prior art is capable of continuous automatic alignment of multiple projectors. While visible content is being displayed (e.g. a video or image), calibration patterns are also displayed, and are photographed by one or more cameras. The calibration patterns may be in the infrared spectrum and thus not distracting to viewers of the desired projected content. Alternatively, the calibration patterns may be embedded in images of the projected content, preferably in such a way that the viewer is unable to perceive or be distracted by the calibration patterns. This latter approach has the advantage that it does not require an additional light source (e.g. infrared emitter), and that a simple and inexpensive camera can be used to photograph the projected pattern.
However, one problem with projecting a calibration pattern in the visible spectrum, including when the calibration pattern is embedded in a projected content image, is that, in a multi-projector system, overlapping projections may reduce the detectability of the calibration pattern. This limits the regions of the projection surface that can be used for calibration, which reduces the accuracy of the alignment procedure.